The rate of ocean‐crust production exerts control over sea level, mantle heat loss, and climate. Different strategies to account for incomplete seafloor preservation have led to differing conclusions ...about how much production rates have changed since the Cretaceous, if at all. We construct a new global synthesis of crust production along 18 mid‐ocean ridges for the past 19 Myr at high temporal resolution. We find that the global production rate during 6–5 Ma was only 69%–75% of the 16–15 Ma interval. The reduction in crust production is mostly due to slower seafloor spreading along almost all ridge systems. While the total ridge length has varied little since 19 Ma, some fast‐spreading ridges have grown shorter and slow‐spreading ridges grown longer, amplifying the spreading‐rate changes. Our production curves represent a new data set for investigating the forces driving plate motions and the role of tectonic degassing on climate.
Plain Language Summary
The question of whether the speeds of tectonic plates vary over time is controversial but has big‐picture implications for our understanding of the forces inside the Earth that drive the plates, the role of volcanoes in controlling climate change over millions of years, and the rise and fall of sea level. At mid‐ocean ridges, two plates move apart, and the volcanic rocks that comprise the ocean crust are created. Magnetic minerals in the rocks record their age of formation and therefore the relative speeds of the diverging plates. However, this record is incomplete because seafloor is destroyed at subduction zones. We used the preserved seafloor magnetic record to calculate diverging plate speeds over the past 19 million years. We find that the relative plate speed at almost all divergent plate boundaries has slowed down, with a major inflection point at 15–16 Myr. As a result, the rate at which new ocean crust is created also slowed down, by roughly 35%. We speculate that there is not a single explanation for the nearly global slowdown in plate speeds but rather several unrelated tectonic events, such as the emergence of the Andes.
Key Points
A new global synthesis of seafloor‐spreading rates at high temporal resolution is presented
Since 15 Ma, spreading rate decreased along 15 of 18 major ridge systems, with a total global reduction of 37%
High‐resolution reconstructions, new data for eastern Pacific, and astronomical ages allow more detail than previous studies
Prolonged standing at work has been shown to be associated with a number of potentially serious health outcomes, such as lower back and leg pain, cardiovascular problems, fatigue, discomfort, and ...pregnancy-related health outcomes. Recent studies have been conducted examining the relationship between these health outcomes and the amount of time spent standing while on the job. The purpose of this article was to provide a review of the health risks and interventions for workers and employers that are involved in occupations requiring prolonged standing. A brief review of recommendations by governmental and professional organizations for hours of prolonged standing is also included.
Based on our review of the literature, there seems to be ample evidence showing that prolonged standing at work leads to adverse health outcomes. Review of the literature also supports the conclusion that certain interventions are effective in reducing the hazards associated with prolonged standing. Suggested interventions include the use of floor mats, sit-stand workstations/chairs, shoes, shoe inserts and hosiery or stockings. Studies could be improved by using more precise definitions of prolonged standing (e.g., duration, movement restrictions, and type of work), better measurement of the health outcomes, and more rigorous study protocols.
Use of interventions and following suggested guidelines on hours of standing from governmental and professional organizations should reduce the health risks from prolonged standing.
Valdivia Bank (VB) is a Late Cretaceous oceanic plateau formed by volcanism from the Tristan‐Gough hotspot at the Mid‐Atlantic Ridge (MAR). To better understand its origin and evolution, magnetic ...data were used to generate a magnetic anomaly grid, which was inverted to determine crustal magnetization. The magnetization model reveals quasi‐linear polarity zones crossing the plateau and following expected MAR paleo‐locations, implying formation by seafloor spreading over ∼4 Myr during the formation of anomalies C34n‐C33r. Paleomagnetism and biostratigraphy data from International Ocean Discovery Program Expedition 391 confirm the magnetic interpretation. Anomaly C33r is split into two negative bands, likely by a westward ridge jump. One of these negative anomalies coincides with deep rift valleys, indicating their age and mechanism of formation. These findings imply that VB originated by seafloor spreading‐type volcanism during a plate reorganization, not from a vertical stack of lava flows as expected for a large volcano.
Plain Language Summary
Oceanic plateaus are large, elevated underwater features commonly formed from volcanic material from a hotspot. Valdivia Bank is a Late Cretaceous oceanic plateau in the southeast Atlantic Ocean formed by volcanism from the Tristan‐Gough hotspot near the Mid‐Atlantic Ridge. The origin and evolution of Valdivia Bank is poorly defined, but new magnetic data suggests the edifice originated through ridge‐centered volcanism, with lateral accretion of crust. This is unlike the evolution of a massive volcano, which would be expected to create a vertical stack of lava flows. Magnetic inversion modeling suggests the plateau was formed by seafloor spreading during the formation of anomalies C34n‐C33r, with the plateau becoming younger from east to west, rather than north‐south as predicted by some hotspot models. Results from International Ocean Discovery Program Expedition 391 paleomagnetism and biostratigraphy confirm the anomaly interpretation.
Key Points
Valdivia Bank is characterized by quasi‐linear magnetic anomalies that are parallel to the inferred paleo‐Mid‐Atlantic Ridge
Magnetic anomalies imply that the plateau becomes younger E‐W consistent with formation via seafloor spreading during anomalies C34n‐C33r
Rift valleys, division of C33r, and anomaly curvature imply complex ridge tectonics and a ridge jump
The capacity of oceanic crust to record geomagnetic polarity reversals makes sea‐surface magnetic anomalies an essential tool to study plate tectonics. The anomalies are usually well‐defined at ...magmatic spreading centers, but are distorted and eventually disappear on magma‐poor mid‐ocean ridges such as the ultraslow Southwest Indian Ridge (SWIR), making their interpretation difficult. We attribute the variability of the SWIR sea‐surface magnetic anomalies to the alternance of magmatic spreading and detachment faulting. A three‐layer magnetic model is used to simulate the influence of such an alternance on the sea‐surface magnetic anomalies. Conversely, observed magnetic profiles at the SWIR are modeled to unravel their off‐axis crustal structure and past mode of spreading. The intruding gabbro bodies on the footwall of detachment faults play a major role in explaining the variability of sea‐surface magnetic anomalies at slow and ultraslow spreading ridges.
Plain Language Summary
Marine magnetic anomalies result from the capacity of oceanic crust to record the ambient geomagnetic field polarity at the time of its formation at mid‐ocean ridges. These anomalies are usually well‐defined on magmatic crust formed at faster spreading ridges, but they are more elusive on magma‐poor crust of slower spreading ridges. Previous magnetic studies on the ultraslow spreading Southwest Indian Ridge show magnetic anomalies that are hardly identifiable on some segments. We investigate the hypothesis that the variability of the marine magnetic anomalies on this ridge results from the alternance of two modes of spreading. In the first one, seafloor spreading is achieved through the formation of magmatic crust, in the second one through detachment faulting which exhumes mantle rocks and intrusive gabbro bodies in a magma‐poor environment. Serpentinized mantle does generally not record accurately the geomagnetic polarity, so magnetic anomalies over the detachment faults rely on the intrusive gabbro bodies, capable to record the magnetic polarity. The quality of the magnetic signal depends on the abundance of these gabbro bodies, which in turn reflects the local degree of magmatism. We use forward modeling to simulate and confirm these hypotheses and test their reliability on observed magnetic profiles.
Key Points
The variability of sea‐surface magnetic anomalies at ultraslow spreading centers reflects different degrees of magmatism along the ridge
Gabbro bodies intruding the footwall of detachment faults partially record the magnetic polarity when this mode of spreading is active
Sea‐surface magnetic anomalies may represent a tool to estimate the spreading mode at ultraslow spreading centers
Gravity models are powerful tools for mapping tectonic structures, especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment. We combined new ...radar altimeter measurements from satellites CryoSat-2 and Jason-1 with existing data to construct a global marine gravity model that is two times more accurate than previous models. We found an extinct spreading ridge in the Gulf of Mexico, a major propagating rift in the South Atlantic Ocean, abyssal hill fabric on slow-spreading ridges, and thousands of previously uncharted seamounts. These discoveries allow us to understand regional tectonic processes and highlight the importance of satellite-derived gravity models as one of the primary tools for the investigation of remote ocean basins.
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